Abstract
Gram-negative bacteria are contained by an envelope composed of inner and outer-membranes with the peptidoglycan (PG) layer between them. Protein translocation across the inner membrane for secretion, or insertion into the inner membrane is primarily conducted using the highly conserved, hourglass-shaped channel, SecYEG: the core-complex of the Sec translocon. This transport process is facilitated by interactions with ancillary subcomplex SecDF-YajC (secretion) and YidC (insertion) forming the holo-translocon (HTL). This review recaps the transport process across the inner-membrane and then further explores how delivery and folding into the periplasm or outer-membrane is achieved. It seems very unlikely that proteins are jettisoned into the periplasm and left to their own devices. Indeed, chaperones such as SurA, Skp, DegP are known to play a part in protein folding, quality control and, if necessary degradation. YfgM and PpiD, by their association at the periplasmic surface of the Sec machinery, most probably are also involved in some way. Yet, it is not entirely clear how outer-membrane proteins are smuggled past the proteases and across the PG to the barrel-assembly machinery (BAM) and their final destination. Moreover, how can this be achieved, as is thought, without the input of energy? Recently, we proposed that the Sec and BAM translocons interact with one another, and most likely other factors, to provide a conduit to the periplasm and the outer-membrane. As it happens, numerous other specialized proteins secretion systems also form trans-envelope structures for this very purpose. The direct interaction between components across the envelope raises the prospect of energy coupling from the inner membrane for active transport to the outer-membrane. Indeed, this kind of long-range energy coupling through large inter-membrane assemblies occurs for small molecule import (e.g., nutrient import by the Ton complex) and export (e.g., drug efflux by the AcrAB-TolC complex). This review will consider this hypothetical prospect in the context of outer-membrane protein biogenesis.
Highlights
The primary function of the outer-membrane is to protect the bacteria from the harsh surrounding environment
The internal leaflet of is made up of phospholipids, but in bacteria with an outer-membrane the external leaflet of the bilayer is exclusively glycolipids. Proteins within this outer-membrane environment primarily consist of β-barrels
These movements depend on the interaction between two highly conserved residues at the center of the proton channel in SecDF (Furukawa et al, 2018)
Summary
The Gram-negative bacterial cell envelope is typically composed of inner and outer-membranes with the periplasm sandwiched in between. The inner membrane is the first membrane in contact with the cytoplasm and is central to energy conservation, lipid biosynthesis, protein secretion and transport. The inner membrane is necessary for the proton motive force (PMF), which consists of an electrical ( ) and chemical ( pH) gradient across the inner membrane created by proton transport into the periplasm. Energy conserved in this process can be converted into kinetic movement of several different membrane proteins to assist in transport across the membrane, or to drive energetically unfavorable reactions such as the phosphorylation of adenosine di-phosphate (ADP) to adenosine tri-phosphate (ATP) by ATP synthase. Proteins within this outer-membrane environment primarily consist of β-barrels
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
